Common options for measuring temperature include the Resistance Temperature Detector (RTD), which has a metallic element possessing a positive temperature coefficient of resistance (decreasing resistance with increasing temperature), or a thermistor, which has a metal oxide element exhibiting a negative temperature coefficient (increasing resistance with increasing temperature). Unlike an RTD, thermocouples can achieve high accuracy without any restrictions on target immersion depth relative to the sensor diameter or length. Unlike a thermistor, thermocouples produce signals without any pronounced nonlinearity with temperature, thus allowing use over a greater temperature range. Finally, unlike a thermal colorimetric material, additional internal sensors are not necessary in order to obtain the signal. These factors make thermocouples particularly useful when used on medical devices, because they have superior capability for measuring temperature in a small or confined area. Accurate measurements of tissue or body fluids temperature are critically important during many medical procedures, but introduction of temperature measurement capability at a specific, controllable location during the procedure is often difficult and sometimes impossible. Application of a thermocouple device directly on a medical device solves this problem, removes uncertainty, during the procedure, about the exact location of the probe, and reduces the number of instruments that must be introduced or used during the medical procedure.
Thermocouples use two leads formed of dissimilar materials, for example one lead formed of constantan and the other formed of copper, that are joined at one end to form a thermocouple junction. The thermocouple junction produces a voltage, representative of the temperature, and that voltage varies as the thermocouple is exposed to various temperatures. Conventional thermocouples are often formed by joining together a pair of dissimilar metal wires, the metals having been chosen so that a voltage is observed depending on the size of the temperature difference between the joined and free ends of the pair.
Thermocouples can be used for both temperature measurement and temperature control by heating the thermocouple junction. The thermocouple effect is where a temperature differential can be converted directly into electrical energy, with the amount of electrical energy so generated providing a measurement of the temperature. The observed voltage then provides an estimate of the temperature differential along the length of the pair of wires according to standard equations well known to those of ordinary skill in the art.
Conversely, if a voltage is applied to a thermocouple, a temperature differential is created between the junction and the free ends of the two elements that comprise the thermocouple, with the junction being either cooled or heated depending on the direction of the applied DC current. If a number of such thermocouples are interconnected, a heating and cooling module (e.g., a Peltier module) may be constructed according to methods well known in the art. Several thermocouples that have been interconnected in series are often also commonly referred to as a thermopile.
As useful and versatile as modern thermocouples might be, they suffer from certain disadvantages, among which are that they are generally not suitable for use on flexible/irregular surfaces. Thermocouples are often made of thin wire pairs so that the device responds more quickly to temperature changes, but such a construction can make the thermocouple somewhat fragile. Several inventions have been aimed at demonstrating and solving the problems inherent in placing a temperature-measuring device directly on or in a medical device, but they have drawbacks and technical limitations.
For example, U.S. Patent Publication No. 2005/0257822 to Smith et al., describes printing finely powdered metals onto bedsheets and the like for use in medical scenarios. The metals are formulated into inks and deposited via silk screening. Silk screening is a popular printing method for applying lines to flexible objects. However, the objects must be held against a noncompliant backing during printing, such that silk screening is limited to two-dimensional or planar surfaces. Most medical devices of interest are polymeric, flexible, and three dimensional, rendering the silk screening approach problematic and often unusable. The silk screening method applies pressure to the substrate during the writing process.
U.S. Pat. No. 4,263,921 to Trugillo et al., describes a temperature sensitive endotracheal tube in which a thermistor is mounted to the tube. Mounting the thermistor rather than writing it directly on the tube surface leads to unwanted protrusion on the surface of the device which may prove damaging or uncomfortable upon introduction of the device to measure temperature during medical procedures. Further, there is an increased risk of the temperature sensor becoming loose and getting displaced during the procedure, leading to uncertainty in probe localization and subsequently in temperature monitoring. Another issue with the design is that thermistors, which rely on changes in resistance vs. temperature for a single metal, exhibit more non-linearity with temperature than do thermocouples, leading to measurement errors or the need for complicated correction algorithms during signal processing.
U.S. Pat. No. 4,046,139 to Horn describes a temperature sensor mounted on the cuff of an endotracheal tube, which could be a thermocouple, thermistor or color-sensitive material. A mounted thermocouple or thermistor would have the same drawbacks discussed above. Furthermore, a temperature sensor reliant on color change would be of limited usefulness when embedded in the body, because a method of sensing and transmitting the color, such as a camera or CCD, would also need to be introduced in able to detect real-time temperature changes.
U.S. Pat. No. 5,596,995 to Sherman et al., describes a thermocouple affixed to an end of a catheter. However, another thermocouple junction is formed where the leads are connected to dissimilar metals. Sherman describes a method for compensating for the unintentional second thermocouple formed at this connection. The electronics are formed separately from the device and then mounted to the device, leading to manufacturing challenges particularly if the catheter is very small in diameter; and also leading to the possibilities outlined above that the thermocouple junctions may be inadvertently displaced during the procedure. Like the other cases, a method is not provided that elegantly affixes the metals to the medical device.
Methods for applying metallic lines directly to flexible three-dimensional surfaces are very limited. Inkjet, or thermal transfer may be envisioned if appropriate substrate control and manipulation could be effected. Having rheological or thermal properties appropriate for precisely printing the ink formulations pose severe limitations. Another approach is silk screening on a flexible surfaces; however, it requires a backing material, such that printing on three dimensional objects is extremely difficult.
The present invention is directed to overcoming these and other deficiencies in the art.